Conventional crankshaft/scotch yoke mechanisms have been used for many years to convert linear motion into rotary motion and vice versa. In such mechanisms, the reciprocating linear movement of the piston or pistons causes the rotary movement of a crankshaft by causing a crankpin-mounted roller or bearing to move back and forth along a slot formed in a single yoke extending from the piston or pistons (see, for instance, U.S. Pat. Nos. 2,404,906, 2,628,602 and 3,517,652; British Pat. Nos. 112,767, 152,799 and 533,047 and German Pat. No. 584,082). To prevent jamming of the roller or bearing, a spacing must be provided between the roller or bearing and at least one bearing surface of the slot of the yoke. Such a spacing is disadvantageous because it causes backlash (i.e., lost motion between the linear movement of the yoke/piston or pistons, on the one hand, and the rotary movement of the crankpin/crankshaft, on the other hand), which results in brinelling, spalling and/or undesired vibrations. Brinelling, spalling and/or undesired vibrations cause, in turn, wear and material fatigue and deterioration.
U.S. Pat. Nos. 963,449 and 4,339,960 disclose crankshaft/scotch yoke mechanisms which are specifically designed for use in Stirling-type engines. These mechanisms employ a single crankpin and a plurality of slotted yolks. The crankpin is provided with one or more rollers or bearings, each roller or bearing translating its motion to one or more slotted yokes. Because each roller or bearing is captured in a slot of one or more of the yokes, a spacing must be provided between the roller or bearing and at least one bearing surface of the slot or slots in order to prevent jamming. The provision of such a spacing creates the same wear, fatigue and deterioration problems discussed above.